In laboratory practice the necessity of optical., microscopic examination of particles suspended in a fluid (emulsion or suspension) often arises to classify the particles according to their shape and determine concentrations of various fractions. In medical practice, determination of blood cell counts or analysis of urine sediments may constitute such tasks. Several similar examinations are carried out in other fields of industry, e.g. in food processing or pharmaceutical industry.
Such examinations are conventionally performed by filling the sample into a test tube, sedimentation it by centrifuging for a required period, removing the supernatant, homogenizing and staining the sediment if required, and transferring a drop of it to a microscope slide, covering it with a coverslip and evaluating the observed image with human eye through a microscope, and repeating this evaluation on a required number of imaging areas.
The above traditional method is extraordinarily inaccurate, time-consuming and can hardly be automated. Numerous methods have been developed to simplify the conventional method.
According to a known method suitable for investigating particles suspended in a fluid, the sample of considerable volume is passed through a flow-cell and the particles flowing in the sample are optically observed and analysed. Such methods are described in Patent Specifications of No. U.S. Pat. Nos. 4,393,466 and 5,123,055.
Deficiency of the methods known so far is that the moving particles can be observed optically for a very short time only, therefore, owing to the limited time available for classifying each particle, practically there is no way to perform morphological testing.